Producing low resistivity carbon for electrodes



Patented Sept. 25, 1056 PRODUCING LOW RESISTIVITY CARBON FOR ELECTRODES Edwin N. Klemgard, Pullman, Wash, assignor, by memo assignments, to Research Corporation, New York, N. Y., a corporation of New York No Drawing. Application October 6, 1952, Serial No. 313,370

5 Claims. (Cl. 202-13) The present invention relates to a carbon of low electrical resistivity and a method of producing the same. It is the object of this invention to produce metallurgical carbon having a specific electrical resistivity below .002 ohm per inch cube and containing less than 0.8 per cent of ash constituents, from natural coal and coal tar pitch or asphaltic pitch.

The demands in the metallurgical industry for high purity, low resistivity electrode carbon are increasing rapidly. In the production of aluminum by the electrolysis of a solution of alumina in fused cryolite, for example, about 0.6 pound of electrode carbon is consumed for each pound of aluminum produced. At the present rate of aluminum production, the requirements for electrode carbon is tremendous. The customary practice of the aluminum industry is to make use of petroleum coke mixed with coal tar pitch in either prebaked electrodes or in the self baking electrodes utilized in the Soderberg process.

Petroleum coke usually will contain not more than the following percentages by weight of mineral impurities which would tend to contaminate the aluminum: sulphur 2.5%; silicon 0.06%; iron 0.06%; calcium 0.12% and sodium 0.12%. The petroleum coke has a low specific resistivity, about 0.0019 ohms per inch cube at room temperature and has a density of about 2.00 or higher. These characteristics have made petroleum coke the source usually preferred for the electrode carbon in thealurninum industry. However, with modern improvements in the art of petroleum refining, the production of petroleum coke is tending to decrease while the demands of the metallurgical industry for high purity electrode carbon is increasing rapidly. The many attempts in the past to produce electrode carbon, using natural coal as a source, have not resulted in any substantial commercial acceptance despite the increased price of petroleum coke.

The present invention concerns itself with a process, using natural coal as a principal material source, for producing an electrode carbon which compares favorably to petroleum cokein mineral impurity content, electrical resistivity, density, strength, and thermal conductivity. Specifically the object of this invention is to prepare low resistivity metallurgical carbon by providing, through selection and beneficiation, a raw coal having an ash content of about 2% or less, crushing this raw coal to a fine powder, then treating the powder, if necessary, with dilute mineral acid such as hydrochloric acid to leach out objectionable minerals and to bring the total ash to; about 1% or less, washing out the water soluble salts formed and remaining after the acid leach, drying the coal, then combining the resulting raw purified coal with a soft coal tar pitch or asphaltic pitch in proportions of at least about 40% by weight of pitch and not more than about 60% by weight of the purified raw coal, driving oil the volatile materials and calcining the mixture, then bringing the calcined mixture to a temperature above 1500 degrees C. but below about 2100 degrees C. (gra'phitization temperature), in the absence of air for a brief time (about 15 minutes), thereby producing an electrode carbon having a specific resistivity of the order of .002 ohm per inch cube and a total ash content of below 0.8 per cent.

The invention is not limited as to the particular source of coal. In the case of certain naturally occurring low ash coals, for example, certain coals from Bell and Franklin Counties in Kentucky, and Walker County in Alabama, it is possible to crush such coals to a fine powder and proceed without beneficiation, to clean by treating and washing, and prepare the coal for mixture with the coal tar pitch. With other coals having a higher proportion of ash constituents, the first step is to reduce the ash constituents to 2% or less by the well established sink and float procedures, utilizing any suitable heavy density media, such as ferrosilicon, zinc chloride solution, or mixtures of carbon tetrachloride and petroleum napthas.

The selected or beneficiated coal of 2% or less ash constituents is next ground to a fine powder, fine enough to pass through a hundred mesh screen. This powder is treated with an acid such as dilute hydrochloric acid to leach out various minerals and reduce the total ash content to less than 1% by weight. About one half pound of hydrochloric acid is required for each pound of ash forming mineral leached out. The coal is then washed to remove the salts formed and the coal is dried.

The dry powdered purified coal, containing less than one per cent ash constituents is next mixed with either coal tar pitch or'petroleum asphalt (asphaltic pitch) in proportions ranging from 60 parts to 25 parts coal, and 40 parts to parts pitch by Weight. The mixture is first heated slowly to a temperature above 200 degrees C. up to 600 degrees C. driving off most of the volatile materials. The temperature is then raised to about 1000 degrees C. for calcining. The volatile materials are re circulated over the cold coal to condense and re-use as much as possible of the pitch, thus cutting down the total pitch requirement.

The calcined mixture is not of sufficient loW resistivity even after heating for several hours, to provide an adequate carbon substitute for petroleum coke. Its specific resistivity will in general be at least twice that of the petroleum coke. I have found that, quickly raising the temperature of the calcined mixture in the absence of air, to a temperature of 1600 degrees C. and above, but below graphitizing temperature of 2100 degrees C., and holding it at this temperature for a brief time, effects a thermal agitation that causes the rapid orientation of the carbon atoms in the purified coal in such a manner that a new carbon network is established in situ. The carbon network is not similar to the carbon orientation in normal graphite. The specific resistivity is somewhat less than .0026 ohm per inch cube and generally of the order of .0019 ohm per inch cube and lower. The thermal conductivity is not nearly so high as that of artificial graphite formed from carbon products, for example, by the well known Acheson electric furnace process wherein the carbons are heated to temperatures in excess of 2100 degrees C. for long periods of time. The specific resistivity does not approach that of present day high quality Acheson graphite electrodes, which is of the order of .00032 ohm per inch cube. it does however, compare fairly well with good amorphous carbon (petroleum coke) electrodes which have specific resistivities of .002 ohm per inch cube down to .00124 ohm per inch cube.

The length of time the calcined mixture is held at a temperature above 1600 degrees C. but below 2100 degrees C. can be very short. From one-fourth minute to thirty minutes is ample. Longer maintenance of the mixture at such a temperature apparently does not work any appreciable benefit or detriment.

The heating to 1600 degrees C. is best done in a high frequency, electric induction furnace. The material can be brought up to the desired temperature quickly and this type of furnace lends itself readily to the exclusion of air.

The following specific example of the making of metallurgical carbon will serve to illustrate the invention. A quantity of anthracite coal from Whatcom County in the State of Washington was used. Analysis of this coal on an air dry basis was:

Apparent density 1.405

Per cent moisture 0.80 Per cent volatiles 4.29 Per cent ash 4.18

Per cent fixed carbon 90.73

The coal was crushed to pass through a /2 inch mesh screen. This crushed coal was then stirred together with a mixture of carbon tetrachloride and petroleum naptha having a density of 1.408 at 25 degrees C. After standing for one hour, 36.6 per cent by weight of the coal floated and was removed by mechanical skimming. (This floated coal was analysed and found to contain 1.60 per cent, by weight, of ash constituents.

The floated coal was then ground in a high speed pulver'izer, to pass through a 100 mesh screen. The powdered coal was then placed in an open acid resistant vessel, and an equal weight of an aqueous solution containing 7.29 per cent of hydrochloric acid was added while stirring. The coal-acid mixture was agitated with air for a period of 48 hours, being held during this time at a temperature varying from 50 degrees C. to 80 degrees C. It was found that about 0.50 pound of hydrochloric acid was required for each pound of ash forming minerals leached from the powdered coal by this method. The coal was allowed to settle and the supernatant acid solution skimmed off. It may be used for further treatment of coal following well known counterflow procedure. The acid leached coal was then washed three times with water, at room temperature, to remove soluble salts remaining in the wet powdered coal. The washed coal was then dried in an oven at 120 degrees C. until substantially free from moisture. It was found to contain less than 1% total ash constituents.

The purified coal was then mixed with soft coal tar pitch having the following approximate analysis:

Melting point deg'rees C 75 Insolubles in carbon disulfide per cent 9 ,Ash content do 0.25 Coking value do 40.0

A mixture of 60 parts by weight of purified powdered coal and 40 parts by weight of soft pitch was then heated in an open vessel to 200 degrees C. and gradually raised to remove the major portion of the volatile matter and avoid frothing when the mixture was calcined. Then the mixture was calcined by holding it at 1100 degrees to 1200 degrees C. for six hours. Samples of the calcined mix were tested and were found to have a specific resistivity of 0.00412 ohm per inch cube.

The calcined mixture was then charged into a high frequency electric induction furnace and held in the absence of air at a temperature of 1600 degrees C. to 1800 degrees C. for fifteen minutes. On cooling to atmospheric temperature the specific resistivity was determined and found to be .00174 ohm per inch cube.

Other samples of the purified powdered coal were heated under like conditions to the same temperatures,

with only 20 parts by weight of pitch added to 80 parts by weight of coal, and, with no pitch added. The specific resistivity of these samples varied from .00315 to .00348 ohm'per inch cube.

Maintenance of the calcined mixtures at temperatures between 1600 degrees C. and 2000 degrees C. for as much as ten hours did not alter the specific resistivity of the product to a measurable extent. It is evident therefore that the desired characteristics of the carbon are obtained by a very short heating time at temperatures between 1600 degrees C. and 2100 degrees C.

Experiments were carried out in which the final heating was not carried as high as 1600 degrees C. The desired low specific resistivity was not obtained. It appears therefore that in order to obtain a satisfactory carbon substitute for petroleum coke in metallurgical electrodes, the composition to be calcined has to be re stricted to at least about 40 per cent pitch and about 60 per cent purified coal. The final heating in the absence of air of the calcined mixtures is critical. A minimum of 1600 degrees C. must be reached. Heating to temperatures as high as 2100 degrees C. for any appreciable time must be avoided. Suo'h heating is known to result in graphitization.

While the use of high frequency electric induction heating is common in industry today, this particular type of heating is of particular advantage in the present invention, because it provides a way of quickly heating the calcined mixture to the critical minimum temperature. Once this temperature is reached, the product can be removed with a resultant economy in heating.

The nature and advantages of the invention are believed to be apparent from the foregoing description.

The invention claimed is:

1. The process for manufacturing electrode carbon having a specific resistivity of less than .0027 ohm per inch cube which comprises the steps of finely dividing a coal having less than 2% ash constituents, leaching the coal with an aqueous solution of mineral acid, washing the coal to remove soluble salts, drying the coal, combining the dried coal with pitch in proportions of about 60% coal to about 40% pitch by weight, heating the mixture of coal and pitch gradually to a temperature of the order of 1000 degrees C. to 1200 degrees C. and calcining the mixture at such temperature in the presence of air to drive off volatile material, then bringing the product in the absence of air to a temperature above 1600 degrees C. and below 2000 degrees C.

2. The process of manufacturing electrode carbon having a specific resistivity of less than .0027 ohm per .inch cube from raw coal having less than 2% ash and coal tar-pitch which comprises first pulverizing the coal,

then treating the pulverized coal with an aqueous solution of hydrochloric acid and washing out the formed soluble salts, then drying the coal, thereafter mixing the coal tar pitch in proportions of about 40% pitch and 60% coal by weight, then heating and calcining the mixture in air and finally raising the temperature of the calcined mixture in the absence of air to above 1600 degrees C. but below graphitizing temperature, then .cooling the mixture.

ing the mixture in the presence of air until the volatile products are removed, then subjecting the calcined product to thermal treatment in the absence of air in a high frequency electric induction .furnace at a temperature above 1600 degrees C. but below graphitizing temperature.

4. The process for manufacturing carbon equivalent in the property of electrical resistivity to petroleum coke, comprising pulverizing raw coal having ash forming constituents of less than 2%, mixing the pulverized raw coal with a. pitch selected from the group of coal tar pitch and asphaltic pitch in proportions by weight of 40% to 75% pitch and 60% to 25% coal, calcining the mixture in air at a temperature of about 1000 degrees C. to 1200 degrees C., then heating the calcined product in the absence of air by high frequency electric induction to a temperature above 1600 degrees C. but below 2000 degrees C.

5. The process for manufacturing carbon equivalent in the property of electrical resistivity to petroleum coke, comprising pulverizing raw coal having ash forming constituents of less than 2%, mixing the pulverized raw coal with a pitch selected from the group of coal tar pitch and asphaltic pitch in proportions by Weight of 40% to 75% pitch and 60% to 25% coal, calcining the mixture in air at a temperature of about 1000 degrees C. to 1200 degrees C. then heating the calcined product in the absence of air to a temperature above 1600 degrees C. but below 2000 degrees C.

References Cited in the file of this patent UNITED STATES PATENTS 1,317,497 Hinckley Sept. 30, 1919 1,317,498 Hinckley Sept. 30, 1919 1,517,819 Beer Dec. 2, 1924 1,982,821 Marsden Dec. 4, 1934 2,148,133 Reuscher et a1. Feb. 21, 1939 2,653,878 Sejersted Sept. 29, 1953 FOREIGN PATENTS 11,684 Great Britain July 15, 1887 of 1886 17,731 Great Britain June 10, 1915 of 1913 OTHER REFERENCES Mantell: Industrial Carbon, 2nd edition, pages 242, 261, 262, 270, D. Van Nostrand Co., Inc. (1946). 

1. THE PROCESS FOR MANUFACTURING ELECTRODE CARBON HAVING A SPECIFIC RESISTIVITY OF LESS THAN .0027 OHM PER INCH CUBE WHICH COMPRISES THE STEPS OF FINELY DIVIDING A COAL HAVING LESS THAN 20% ASH CONSTUTUENTS, LEACHING THE COAL WITH AN AQUEOUS SOLUTION OF MINERAL ACID, WASHING THE COAL TO REMOVE SOLUBLE SALTS, DRYING THE COAL, COMBINING THE DRIED COAL WITH PITCH IN PROPORTIONS OF ABOUT 60% COAL TO ABOUT 40% PITCH BY WEIGHT, HEATING THE MIXTURE OF COAL AND PITCH GRADUALLY TO A TEMPERATURE OF THE ORDER OF 1000 DEGREES C. TO 1200 DEGREES C. AND CALCINING THE MITURE AT SUCH TEMPERATURE IN THE PRESENCE OF AIR TO DRIVE OFF VOLATILE MATERIAL, THEN BRINGING THE PRODUCT IN THE ABSENCE OF AIR TO A TEMPERATURE ABOVE 1600 DEGREES C. AND BELOW 2000 DEGREES C. 